A marine charity has offered a £5,000 reward to find the thugs behind a spate of porpoise shootings off the North East coast.

A police investigation has been launched after reports the protected species have been shot and clubbed to death.

In recent weeks more than 30 porpoises have been washed up dead on beaches between the Scottish border and Humberside.

The latest report, of a porpoise being shot off Newbiggin, Northumberland, was made on Wednesday.

Two others were found with what appear to be bullet holes in their heads at Longhoughton and Beadnell, Northumberland, in the past two weeks.

Now marine conservation charity The Deep, which runs an aquarium in Hull, has offered a £5,000 reward to anyone who gives information leading to the prosecution of those responsible. Colin Brown, chief executive of The Deep, said: "We can't stand by and let this slaughter happen on our doorsteps."

Under the Wildlife and Countryside Act 1881, porpoises, along with whales and dolphins, are protected species and anyone found guilty of ill-treating them could face a six-month prison sentence.

Andrew Tait, North East representative of the Seawatch Foundation, which monitors marine mammals, said that in the past fortnight he has seen four dead porpoises on beaches between Beadnell and Whitley Bay.

He said: "Two of them, at Druridge and Boulmer, were strandings and had definitely died from natural causes. But the other two had holes in their head and looked as if they had been shot."

Police in Humberside are also investigating reports that more than 30 porpoises have been found either shot or battered to death.

They believe the killers may be fishermen who have accidentally caught porpoises in their nets.

Decapitated porpoises discovered off the North East coast in recent months were thought to have become the victims of seagulls after they were killed by trawler fleets.

It is also thought sharks swimming off the coast of Northumberland could be responsible for some of the deaths.

Mike Green, watch officer for HM Coastguard, covering the North East coast, said today: "We are getting reports of dead porpoises from Scarborough to Newbiggin."

He urged people who find beached dolphins or porpoises to call him on 0797 403 0356.